This invention relates to the coating of molded products, and more particularly to the spraying of a powder coating onto the interior surface of a mold prior to the use of that mold in the forming of a molded product.
It is now common practice to mold sheet molding compound, often referred to as "SMC," within pressurized molds. Such parts are commonly manufactured from bulk resin material having very substantial fiber content. This molding technique is widely used for high volume production of large plastic parts. A characteristic of parts molded from sheet molding compound, though, is that they have very poor surface quality. In order to improve that surface quality, it has been suggested to coat the mold interior surface with a powder coating which, when the mold is subsequently filled with sheet molding compound and that compound is cured, forms a surface coating on the resulting product.
The powder coating is a powdered thermosetting resin which is generally sprayed onto the interior heated surface of the mold before the mold is filled. The temperature of the mold, which is generally on the order of 250.degree. F. to 350.degree. F., melts the powder, causing it to flow and then to partially set or cure. When a sheet molding compound or bulk resin is subsequently added to the mold and the mold is heated to cure the bulk resin, the resin of the coating, as well as the bulk resin, cure so as to interact the two resins into one integral product having a smooth, attractive outer surface or skin.
This process and technique for molding sheet molding compound and for applying a powder coating skin to the resulting product is described in U.S. Pat. No. 4,205,028. Equipment for practicing this process is disclosed in U.S. Pat. No. 4,472,451.
In the spraying of powder coating materials, a problem occurs in confining the sprayed powder to the mold cavity and in preventing the buildup of powder on hot surfaces of the mold external to the mold cavity. Such molds commonly have shear edges at the interface of the male and female halves of the mold, and it is these shear edges which are particularly vulnerable to becoming coated with a buildup of sprayed powder material. In the above-identified U.S. Pat. No. 4,472,451 there is disclosed apparatus for protecting the shear edge of the mold from becoming coated with powder directed onto the interior mold surface. That apparatus comprises a mask in the form of a shield having a knife edge placed in close proximity to, but spaced from, the shear edge of the mold. That knife edge and a flexible seal spaced from the knife edge, together with a surface upon which the flexible seal and knife edge are mounted, define a conduit through which air is directed to a slot defined between the shear edge of the mold and the knife edge of the shield. This airstream protects the shear edge of the mold from becoming coated and prevents undesirable buildup of powder coating material on the shear edge of the mold.
The use of the apparatus disclosed in the above-identified U.S. Pat. No. 4,472,451 requires a seal between the mold and the mask in order to effectively protect the shear edge of the mold from oversprayed powder. In the absence of a seal, air leakage occurs, and an airstream is not properly directed at sufficient velocity onto the shear edge. This can result in powder building up upon the shear edge. That buildup of powder could then result in failure of the mold to properly close and seal.